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Triticum Agricolam

6th Order Port Area Experiment

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1 hour ago, Smoove said:

what was the difference in rise between the 3 styles?
 

Here is the impedance graph of the three styles at different power levels:

5995376b41e9f_LargePortImp.PNG.16a85fd52eda3bd278e640fccbcdbb1b.PNG

5995376d5b5a8_SmallPortImp.PNG.d828dc7477fd4ba3507cc4d68dee150b.PNG

5995376c465a0_PluggedImp.PNG.2c362c86a3654f2767c9d6ed81612bf1.PNG

What jumps out at me is how as power is increased into the small rear port config the impedance spike around 23 hz diminishes, and the impedance around the 32 hz tuning increases.  The typical three spike impedance graph of a 6th order system begins to look like the two spike graph of a 4th order system.  

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22 minutes ago, Triticum Agricolam said:

Here is the impedance graph of the three styles at different power levels:

5995376b41e9f_LargePortImp.PNG.16a85fd52eda3bd278e640fccbcdbb1b.PNG

5995376d5b5a8_SmallPortImp.PNG.d828dc7477fd4ba3507cc4d68dee150b.PNG

5995376c465a0_PluggedImp.PNG.2c362c86a3654f2767c9d6ed81612bf1.PNG

What jumps out at me is how as power is increased into the small rear port config the impedance spike around 23 hz diminishes, and the impedance around the 32 hz tuning increases.  The typical three spike impedance graph of a 6th order system begins to look like the two spike graph of a 4th order system.  

Like you were saying, it probably has something to do with the small port causing so much port compression at the higher power levels that the chamber starts to act like it's sealed. It's showing up with the spl graphs and the impedance plots. I would be curious to see if the front port velocity and driver excursion are similar between the small port setup and the 4th order setup. I wouldn't be surprised if they were honestly.

Edited by DLHgn

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The in-vehicle testing was just a repeat of what I measured outside.  I'm not going post the graphs showing each configuration at different power levels since it pretty much shows the same thing as the outdoor measurements just with cabin gain added in.  If someone really wants to see it just say so and I'll post them up.  

Here is the graph showing the four combinations at max power:

In-Vehicle.png.874d610ede27601b488cfcccf33749bf.png

The yellow line is the large port config, the pink line is the 4th order config, the blue line is the small port config, and the green line is the low tuned small port config.

Since cabin gain boosts low frequency output and the large port box performed better than the rest down low it is now the highest peak output config.  

As kind of a side note, here is a comparison between the outdoor measurement of the large port config and the in-vehicle measurement:

5995a92806943_CabinGain.png.910b9f4979302ffc9f9cc9ddeb7ab534.png

You can see the cabin gain really has a big effect especially between 30-40 hz.  Its boosting output around 15 dB!

 

 

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I can see how some one might argue that the small LT port has better response below 23ish Hz for little sacrifice from 23-40Hz.  Cabin gain does so much for the entire playing range - especially 20-50Hz - that some folks playing below 20Hz may desire the 'cone control' and lower impedance values (resulting in increased power delivery to the subs) at those low frequencies.

The lower impedance with the small port at low frequency is a bit counter intuitive to me.  I would expect port compression to limit power.  Not quite whats happening here.  Awesome test data!

 


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Very awesome data! I would have chosen the large port area even before seeing the in car measurements. That 2pi outdoor response is nice!

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Triticum

Do you think shrinking the low-tuned chamber (while keeping the large port area) would increase cone control at low frequencies?  Would shrinking the low side give you the benefits of cone control without having to shrink the port (thereby increasing port velocity)?  


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On 10/30/2017 at 1:16 PM, WalledSonic said:

Triticum

Do you think shrinking the low-tuned chamber (while keeping the large port area) would increase cone control at low frequencies?  Would shrinking the low side give you the benefits of cone control without having to shrink the port (thereby increasing port velocity)?  

Sorry I didn't respond sooner.  

The main change shrinking the low side chamber is going to have (assuming everything else stays the same) is its going to raise the low side tuning frequency quite a bit and the high side tuning frequency slightly. 

Here is what Hornresp shows:

Capture1.PNG.77d702c525450d7c94d26fb6651f1640.PNG

This is frequency response, the red line shows the reduced low side chamber, and the red line is the unmodified enclosure.

Below is cone excursion:

Capture2.PNG.8d5058942166e41269895f77a34e0083.PNG

The grey line is the unmodified enclosure, and the red line is the reduced low side chamber box.  

Reducing the low side chamber volume does reduce excursion a little bit between the tuning frequencies, but it also raises the frequency at which the port unloads, so excursion increases on the low end faster. 

 

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So its been about six months since I did anything with this experiment.  I had several things left to test still back in August and I didn't get a chance to finish them, until now.  

It was suggested that I build a second test setup using lower tuning than my first one.  The first one was tuned to about 32 & 53 hz, the second setup is tuned to about 25 & 52 hz.

For this round of testing I had three configurations, one with a larger rear port (17.5 cu in per cube), one with a small rear port (6 sq in per cube), and one with a plugged rear port (effectively a 4th order bandpass).  All three have the same relatively large front port (18 sq in per cube).  For each of the three configurations I measured impedance at 55 different frequencies between 10-100 Hz and at two different power levels.  I then measured the SPL output in-vehicle for each of the three different configurations at 6 different power levels.  For my previous measurements I also measured the halfspace (outdoor) SPL output, but since there is still snow on the ground here I decided to just to do the in-vehicle measurements.  The outside measurements pretty much matched up with the in-vehicle measurements (plus cabin gain) last time, so I think doing both is kind of redundant anyway.  

First off, here are the impedance measurements for the sealed rear chamber configuration:PP4O.PNG.5d88d09fb6752570915d70191471a9a1.PNG

This shows a pretty typical 4th order impedance curve.  It has two peaks with a dip between them, the dip is at the tuning frequency.  You can see that at the higher power level the tuning frequency drops slightly and the bottom of the dip at tuning rises a little bit, indication a bit of compression.  

Now here is the large rear port 6th order impedance:

LPLT.PNG.c32882fd6af0365d750c51f8720c16f6.PNG

 

This is a fairly normal 6th order impedance curve with three peaks and two dips, one at each tuning frequency.  Comparing the low and high power measurements we can see the tuning frequencies shift down slightly and a small increase in impedance at the bottom of both dips indicating a little bit of port compression, much like the 4th order measurement.  

 

Lastly we have the small rear port 6th order configuration:

SPLT.PNG.afd18dbb8ce23cb96ec983cebda32d11.PNG

Like the large port config, we can see that the dips at tuning for the high power measurement are higher, indicating some compression.  However compared to the large port config and the 4th order config the port compression is much more severe.  

 

Lets take a look at the SPL measurements now:

5a792ec1a089d_0dB.png.2b913b2e2cc7f0f20940fa66dae6e17d.png

This is all three configs at the max power I measured at (about 600 watts).  The purple line is the large port config, the red line is the small port config and the green line is the plugged port (4th order) configuration.   I'll get to my analysis of these graphs in my next post. 

 

Edited by Triticum Agricolam
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"Nothing prevents people from knowing the truth more than the belief they already know it."
"Making bass is easy, making music is the hard part."

Builds:

U7qkMTL.jpg  LgPgE9w.jpg  Od2G3u1.jpg  xMyLoO1.jpg  9pAlXUK.jpg

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